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Comic: the future of our beech forests

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Comic: the future of our beech forests

June 10, 2025 Andrew James

Illustrator Andrew James explains Manaaki Whenua research on how climate change might affect rat populations in New Zealand’s beech forests.

Climate change, seeds and rats. By Andrew James. Based on research by Giorgia Vattiato, Adrian Monks et al, Manaaki Whenua. New Zealand’s beech forests are unique places. Home to incredible birds, like mohua (yellowhead. They’re also home to ship rats. Climate modelling shows that New Zealand will be warmer on average in the future. In this warming climate, what will happen to beech forests?

Beech forests cover nearly 3 million hectares of Aotearoa. Growing up to 30 metres tall with small hardy leaves, these giants tranform sunlight into food all year round. Their leaves, sap and wood are food for heaps of different bugs. so provide a smorgasbord of treats for mohua. Their seeds are kai, too. But there's a catch...

Beech tree reproduction is a bit weird. Most years, trees in a few areas flower, but every few years, large numbers of beech trees across a vast area flower together. This is called masting. Masting produces a vast amount of seed, essential for growing the next generation of beech trees. And a food source for native birds–in mast years, birds raise more chicks than in normal years.

But this seed is also a favourite food for rats. Without predator control, a mast leads to a rat boom. Without predator control, many rats compete with birds for food and eat their chicks and eggs. When the mast is over, rat number dip back down to “normal.”

Understanding masting is vital for effective predator control–it’s important to keep rat numbers low when there’s an abundance of food. There’s been a lot of research on it already, so we know that a summer warmer than the last can trigger a mast. Manaaki Whenua scientists used existing data to make two models: One for beech seeding based on climate. And one for rat populations based on beech seeding.

They found that in the future, beech trees will probably produce more seeds in normal years, with bigger and more frequent masts. Ship rats will be licking their lips at the prospect of extra food. Rat modelling suggests that with more frequent masts, there will be booms on top of booms. There won’t be enough time between masts for rat populations to subside to “normal.”

We know that mast years are a problem for birds—mohua vanished from Mt Stokes in the Marlborough Sounds after a “mega mast” twenty years ago. The short story is that climate change is good news for rats. Warmer climate. More seeds. More rats. Goodbye birds? Climate change could create a predator paradise–but it doesn’t have to. Predator control works. It can stop rats from overrunning our beech forests and help mohua and other taonga species reclaim their home.